The present invention relates to the measurement of a current waveform, particularly relates to current measurement technique and a current measurement apparatus for measuring high frequency current that flows in a transmission line of an electric circuit which is a load of small impedance.
Heretofore, to measure a current waveform, a method of insetting a resistor of approximately 10 xcexa9 in series with an electric circuit and monitoring potential difference between both ends of the resistor by a voltage probe has been adopted.
The defects of this method are as follows.
(1) As the resistor is inserted, the device under a test (DUT) is processed (destructive measurement).
(2) As the proper impedance of the device under the test is varied by the insertion of the resistor, the proper current cannot be measured (electric invasive measurement).
For another method of measuring a current waveform, there is a current probe. In this current probe, current is measured by surrounding the transmission line with a series of ring transformer, applying a magnetic field generated based upon current to this transformer and monitoring induced voltage hereby generated. The defects of this method are as follows.
(3) Lead wire is required to be added to the device under a test and is required to be through the transformer (destructive measurement).
(4) The proper impedance of the device under the test is varied by the impedance (approximately 20 nH because the length is approximately 20 mm) of the inserted lead wire and the proper current cannot be measured (electric invasive measurement).
(5) The bandwidth of the current probe is approximately 1.5 GHz and is small (low-time resolution).
A load of small impedance according to the invention is equivalent to a writing element of a magnetic head of HDD for example. The impedance is 10 xcexa920 nH or less and as an operating frequency is increased, the impedance has a tendency to further lower. In the above-mentioned conventional measurement technique, as impedance equivalent to the load or larger than the load is inserted into the device under the test, a current waveform far from the proper current is measured.
For a document related to the technique of this type, Japanese Laid-Open No. 2000-46891 can be given.
As described above, as the conventional measurement technique caused the destructive measurement and the electric invasive measurement, the precise measurement of current that flowed in the transmission line of the electric circuit which was the load of small impedance was impossible. Besides, as the frequency band was small (the time resolution was low), the precise measurement of high frequency current was impossible.
The object of the invention is to provide measurement technique and a measurement apparatus wherein the waveform of current that flows in an electric circuit which is a load of small impedance can be measured in wide bandwidth (at high-time resolution) and at high sensitivity without processing the device under a test (non-destructive measurement) and without having an effect of the impedance of the measurement apparatus upon the device under the test (noninvasive measurement).
To achieve the object, the current measurement technique according to the invention is mainly characterized in that when current that flows in a transmission line of the electric circuit is measured, a magnetooptical device is installed in a magnetic field generated based upon the current that flows in the transmission line, polarized light is made incident on the magnetooptical device and current is acquired based upon the variation of the polarization of reflected light from the magnetooptical device on which the polarized light is made incident.
Besides, the current measurement apparatus according to the invention is mainly characterized in that a light source for radiating polarized light, the magnetooptical device which is installed in a magnetic field generated based upon current that flows in the transmission line and which applies the variation of polarization proportional to magnetization induced by the magnetic field to polarized light and reflects the polarized light when the polarized light radiated from the light source is incident and means for converting the variation of the polarization included in the polarized light reflected on the magnetooptical device to an electric signal and measuring current are provided.
According to the current measurement technique and the current measurement apparatus respectively described above, as the transmission line is not processed and no impedance of the measurement apparatus is applied to DUT, nondestructive and noninvasive current measurement is enabled.
Besides, it is desirable in the current measurement technique and the current measurement apparatus according to the invention that a magnetic field generator for applying a magnetic field to the magnetooptical device is further provided. As a frequency response of the magnetooptical device is enhanced by applying a magnetic field generated by the magnetic field generator to the magnetooptical device, current can be measured in wide bandwidth (at high-time resolution).